<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.5.1/jquery.js"></script>
    <link rel="stylesheet" href="./stylesheets/index.css">
    <title>jquery</title>
  </head>
  <body>
    <div id="box" contenteditable="true"></div>
    <script>
      // 1、获取id为box的jquery元素，并保存在变量container中

      // 2、设置container的边框样式为none，并且背景颜色为red

      box.onfocus = function () {
        // 3、给box元素添加类名为active

      }

      box.onblur = function () {
        // 4、去除box元素的类名active

      }
    </script>
  <body>
</html>